Coral reefs have long been viewed as complex undersea communities, bustling with life. But that vibrant image is more than skin deep, says Amy Apprill.
"Generally, when people think of reefs, they think of corals and fish, but there is much more there," said Apprill, a microbiologist and marine geochemist at Woods Hole Oceanographic Institution (WHOI). "We are zooming in with a microscope to take a higher-resolution look."
That closer look is revealing that coral reefs are teeming with microscopic life--bacteria, algae, viruses, and single-celled organisms called protists. "Coral reefs are microbial hotspots," Apprill said.
Scientists have found that the sediments beneath coral reefs contain 10,000 times more bacteria than the surrounding seawater. And in a new study, Apprill and colleagues identified throngs of bacteria living within coral tissues.
"We are looking at the coral complex like a superorganism," she said: All the various life forms are living together in a web of interactions that contribute to the functioning and well-being of the whole coral reef community.
A decade ago, scientists began applying this superorganism perspective to people, looking at the human body as an ecosystem incorporating microbes in symbiotic collaborations. The sheer number of microbial cells in each person, roughly ten times the number of human cells, suggested that microbes play important roles within us and that we couldn't live without them.
Research has shown that specific types of bacteria settle in certain areas of the body such as the gut and respiratory tract, helping us digest food, manufacture chemical compounds we need, catalyze chemical reactions, and prevent infections by other bacteria. The National Institutes of Health started the Human Microbiome Project to take an ecological survey of the microflora living in the body and their role in human health.
Apprill has taken the same approach to coral reefs. "It's a non-intuitive way to look at it," she said. "You look at the coral and don't see these assemblages of life forms that are not visible to the naked eye."
Scientists have long known that corals have symbiotic relationships with photosynthetic algae, called zooxanthellae. They use sunlight to make food for the coral animals in exchange for a stable scaffold to dwell in. Zooxanthellae are densely packed within coral skeletons--one million of them per cubic centimeter.
But during her early graduate work at the University of Hawaii, Apprill saw hints that corals might have symbiotic relationships with bacteria, too. She found that when juveniles of a coral species called Pocillopora meandrina were ready to attach to a reef surface, they were colonized by bacteria. She suspected that the bacteria might be playing a role in the complex process that determines where coral larvae settle on the reefs.
"When we looked at healthy corals, we began to see these really well-established microbial relationships," Apprill said. "Who are these bacteria? Where are they specifically located? What are they doing? Are they helping corals? There were only a handful of studies on coral-bacteria relationships, and so much still to learn. …